Why wouldn't NBC allow "West Wing" star Martin Sheen to appear on ABC's "Who Wants to Be a Millionaire" to drum up money for charity, but will allow "Friends" star Matthew Perry to appear on Fox's "Ally McBeal" to drum up viewers for the ratings-challenged series?

Perry will appear in two back-to-back hours of the show, playing a "cocky, successful, opinionated attorney who wreaks havoc on Ally's personal and professional life," which is what everyone does on "Ally."

Last season NBC told Sheen -- and late-night host Conan O'Brien -- that it did not want them appearing on the "celebrity" editions of ABC's game show, in which the winnings are donated to the star's charity of choice. Sheen and O'Brien backed out just days before the scheduled taping.

An NBC representative pointed out that the "Millionaire" performances were scheduled when that show was still hot and that the feeling was ABC was stocking the program with NBC talent -- although actually just the two men were scheduled for that celebrity edition.

NBC hasn't tried to dissuade other prime-time stars from appearing on competing networks' series this season, the network said. Bradley Whitford of "The West Wing" appeared briefly on the post-Super Bowl broadcast of "Malcolm in the Middle" -- his wife, Jane Kaczmarek, stars as the mom on that sitcom.

And Sheen will show up on son Charlie's ABC sitcom, "Spin City," this season.

But neither is nearly as big a "get" as Perry, who stars on the most watched TV program in America.

Still, the show, which is the first two-hour "Ally," is not going to air during the May sweeps; it will air on April 15 -- a much more important day in the life of "Ally," which this season has suffered some of its worst ratings ever. Fox is yanking it off the air on March 11 to make room for a new series, "The American Embassy." April 15 will be Ally's first night back on the Fox lineup.

The new series is about a single, confused, skinny, pretty career woman who has sex with good-looking strangers in unusual places -- sound familiar?